Warming of fluids has various applications in any number of fields, for example medicine. In the medical field, warming of fluids is desirable during various procedures, particularly in those involving the intravenous administration of fluids to a patient. This issue becomes important given that certain fluids vital to patient resuscitation (such as blood or blood products) require preservation and storage at low temperatures in order to prevent them from spoiling or contamination. Hence administration of such fluids (e.g. packed red blood cells) requires warming them in order to avoid causing hypothermia in the patient receiving it. Other fluids may require warming prior to being intravenously infused in a patient even though said fluids may be stored at room temperature. It is important to note that the human body's normal temperature, which is critical to normal physiologic homeostasis (typically around 37 degrees Celsius), may be significantly higher than room temperature. Therefore, exposure of patients (intravenous or any other route) to therapeutic fluids that are lower than normal body temperature may not only cause significant discomfort, but also have physiologic consequences which can cause adverse clinical effects and unwanted outcomes. Accordingly, several systems, apparatus, and methods are found in the prior art describing different means to warm fluids such as refrigerated blood and other fluids that require intravenous or intraperitoneal administration. Unfortunately, the prior art solutions are riddled with numerous problems that have yet to be properly addressed.
One common problem is the application of non-uniform electric fields to warm a therapeutic fluid such as intravenous (IV) fluid, which result in an inhomogeneous heating of the liquids. Other problems are presented by conduction heating methods, such as methods that pass blood through heated conduits, which are energy inefficient, less portable and slow, and thus impractical in emergency situations. Other more advanced methods include the introduction of microwave heating, but these methods too have been shown to introduce their own challenges. Primarily, it is now well known that simply heating fluids such as blood (i.e. for example by placing a blood bag inside a conventional microwave oven) carries unacceptable risks given that heating blood in this manner does not result in a uniform distribution of heat throughout the fluid being heated. This important issue is a result of the manner in which microwaves are introduced that leads to generation of hotspots, exposing some areas of the fluid being warmed to excess heat. This will not only be undesirable given the non-uniform nature of heating, but can also lead to adverse effects such as damage to components of the fluid being warmed (i.e. damage to red blood cells or protein structure/function).
While some current methods appear to address hotspots created by systems that implement microwave heating means, these systems appear to rely on components and apparatuses that themselves present additional problems; such problems include introduction of additional steps/equipment (cartridges) in the fluid delivery apparatus (i.e. tubing). This disrupts the continuity of the delivery system (by requiring the tubing to be connected to a cartridge) and creates points where error and contamination can occur, hence raising safety and sterilization concerns. The following examples merely illustrate some of the problems found in the prior art.
One application requiring the warming of such fluids prior to administration includes the warming of peritoneal dialysis dialysate prior to intraperitoneal infusion. For example, certain patients with end-stage renal disease require renal replacement therapy for survival. One modality of renal replacement therapy is peritoneal dialysis (PD); a PD catheter is placed in the patients' abdomen and dialysates (either sterile solutions containing fixed amounts of electrolytes, lactate and dextrose or other infusate such as Icodextrin) are infused into the peritoneal cavity. During treatment, the patient's peritoneal membrane is used as a dialysis membrane and excess serum electrolytes and toxins are removed via diffusion into the dialysate. Given the large volume of dialysates needed each time a patient fills their peritoneal cavity (on average between 2.0-2.5 L), this fluid is usually warmed to between 35° C. and 37° C. to avoid patient discomfort and other unwanted side effects of hypothermia given cool fluid is entering the abdomen. The current system used to warm PD dialysates relies on heat conduction. The warming process is highly inefficient and is fraught with excess time and energy wastage. The system requires warming up a large surface of the dialysis machine and relies on conduction of this heat to a PD dialysate bag, which is placed on top of this surface.
While there are reports of patients/dialysis centers using microwave ovens to warm PD dialysate fluid, this practice is not sanctioned by the US Food and Drug Administration (FDA) or manufacturers of PD solutions, given the potential for formation of hot spots during use of conventional microwave ovens. This is in light of the fact that there are several reported studies in the literature noting mere exposure to RF energy is safe and efficient, and does not lead to disturbance of the PD dialysate content or the integrity of the bag. Several publications provide discussion of these issues such as “Control of microwave heating of peritoneal dialysis solutions” by Deutschendorf A F, Wenk R E, Lustgarten J, Mason P., appearing in Peritoneal dialysis international: journal of the International Society for Peritoneal Dialysis. 1994; 14(2): 163-7; “Microwave ovens for heating fluid bags for continuous ambulatory peritoneal dialysis” by Hudson S, Stewart W K, appearing in British medical journal. 1985; 290(6486):1989; “Rapid warming of infusion solution” by Yamada Y, Yasoshima A. appearing in Surgery, Gynecology & Obstetrics. 1985; 160(5): 400-2; and “Microwave warming of peritoneal dialysis fluid” by Armstrong S, Zalatan S J. appearing in ANNA journal/American Nephrology Nurses' Association. 1992; 19(6): 535-9; discussion 40. However, regardless of these reports, significant safety concerns surrounding hotspot generation and non-uniform warming of dialysate, which can result in serious complications, have precluded routine use of general microwave ovens as a means of warming peritoneal dialysate.
Another important area where warming of therapeutic fluids is of significant value is in critical care when either large volume resuscitation is needed (i.e. liver transplantation, trauma from motor vehicle accidents or battlefield injuries) or in the peri-intra-postoperative period. In many cases the latter scenarios are interrelated and in all cases patients can suffer clinically significant hypothermia. Hypothermia, defined as core temperature <36° C. during a procedure, is a common problem in critical care and among surgical patients. In the case of patients undergoing surgery, an incidence of 4% to 72%, and up to 90% has been reported. Intraoperative hypothermia has been associated with significant clinical complications, including risk of cardiovascular adverse effects, issues with hemostasis and perioperative hemorrhage, increased risk of postoperative infection and disturbed drug metabolism. Given these significant complications, many professional societies, such as the Association of periOperative Registered Nurses (AORN), www.aorn.org, and the National Institute for Health and Care Excellence (NICE), www.nice.nhs.uk, have recommendations in place for preventing and treating during the perioperative period. While there are many factors which may contribute to hypothermia the use of un-warmed fluids for intravenous infusion has been deemed to play a major role. While the positive effects of normothermia in these patients has been documented, the role of warming of patients or infused fluids has been mainly studied using incubators and convection methods. “The effects of warming intravenous fluids on intraoperative hypothermia and postoperative shivering during prolonged abdominal surgery” by Camus Y, Delva E, Cohen S, Lienhart A published in Acta Anaesthesiol Scand. 1996 August; 40(7):779-82. “The effects of intravenous fluids temperature on perioperative hemodynamic situation, post-operative shivering, and recovery in orthopaedic surgery” by Hasankhani H, Mohammadi E, Moazzami F, Mokhtari M, Naghgizadh M M. published in the journal Can Oper Room Nurs J. 2007 March; 25(1):20-4, 26-7. Again, these methods are fraught with inefficiency, lack of portability and excess time requirement. Therefore, novel fluid warming technologies which can address hypothermia in the scenarios mentioned will be of significant value. The application of microwave technology has been limited and will be discussed in the next section.
Another important application involves the need for warming of blood and blood products (red blood cell transfusion); a treatment which becomes necessary to maintain the oxygen-carrying capacity in patients with severe anemia, especially those who have suffered major trauma or patients undergoing major surgery. During resuscitation of the latter patients, multiple units of blood products or packed red blood cells (PRBCs) may be administered in a short period of time. Such products or PRBC units are normally refrigerated at low temperatures of 4±2° C. prior to transfusion. The FDA regulation recommends storage temperature in the range of 1° C.-6° C.; “Safe storage” would be considered to be void if the temperature exceeds 8° C. (See for example FDA “Guide to inspections of blood banks,” published by the FDA, Office of Regulatory Affairs Washington. 14 Sep. 1994).
For patients requiring large volumes of blood transfusion, to prevent hypothermia, the PRBCs units must be warmed up rapidly and almost immediately before transfusion. Aside from the inherent energy inefficiency of convection heating methods, using known means that implement conduction, could prove problematic; especially in emergency situations where considerable transfusions are required to be infused rapidly.
Although delays resulting from heating means relying on conduction of heat appeared to have been addressed by microwave heating methods, these systems proved similarly problematic. The use of conventional microwave ovens or other adapted derivatives to warm blood and IV products became popular soon after the introduction of commercial microwave ovens in the mid-1950s and was regularly used up until the 1970s. Such devices offer shorter heating times than the conventional heaters such as those using a water bath, but several reports of complications from overheating of blood products led to abandonment of microwave oven blood warmers. See for example “Danger of overwarming blood by microwave” by Arens J F, Leonard G L published in Jama. 1971; 218(7): 1045-6. Considerable ongoing debates remain regarding the use of these devices (see for example, “Indicators of erythrocyte damage after microwave warming of packed red blood cells” by Hirsch J, Menzebach A, Welters I D, Dietrich G V, Katz N, Hempelmann G. published in Clinical chemistry. 2003; 49(5): 792-9; and “Temperature course and distribution during plasma heating with a microwave device” by Hirsch J, Bach R, Menzebach A, Welters I D, Dietrich G V, Hempelmann G. published in Anesthesia 2003; 58(5): 444-7).
There are several reports that describe the use of various microwave-based techniques to warm blood products, which do not involve heating up a blood bag inside a microwave oven, per se. However, each of these methods is complicated by an apparent inability to avoid hot spots, or use techniques that require the use of a disposable cartridge. The former having the potential to damage or inadequately heat up the fluids; the latter introducing a point of disruption in the delivery of the infusate which can create the potential for clinically significant adverse events such as entry of air, contaminants or infection given that the need for a cartridge breaks the continuous sterile transfusion system (i.e. the tubing connecting the infusate to the patient). In addition, the need for a cartridge adds another layer of cost and complexity which is less desirable. (See for example, “Microwave applications in clinical medicine” by Lantis J C, 2nd, Carr K L, Grabowy R, Connolly R J, Schwaitzberg S D. published in Surgical endoscopy. 1998; 12(2): 170-6; “The limits of bloodwarming: maximally heating blood with an inline microwave blood warmer” by Herron D M, Grabowy R, Connolly R, Schwaitzberg S D. published in The Journal of trauma, 1997; 43(2): 219-26; discussion 26-8; “In-line microwave blood warming of in-date human packed red blood cells” by Pappas C G, Paddock H, Goyette P, Grabowy R, Connolly R J, Schwaitzberg S D. published in Critical care medicine, 1995; 23(7): 1243-50; “The effect of in-line microwave energy on blood: a potential modality for blood warming” by Holzman S, Connolly R J, Schwaitzberg S D. published in The Journal of trauma. 1992; 33(1):89-93; discussion −4; and “Rapid in-line blood warming using microwave energy: preliminary studies.” By Schwaitzberg S D, Allen M J, Connolly R J, Grabowy R S, Carr K L, Cleveland R J. published in Journal of investigative surgery: the official journal of the Academy of Surgical Research. 1991; 4(4):505-10).
Accordingly, there is an unanticipated and significant clinical need, which is inadequately addressed at this time for warming fluids. More specifically, there is a need in the art for a fluid warming technique whereby fluids, such as intravenous (IV) fluids, can be warmed to the desired temperature via a warmer apparatus that avoids the potential complications of localized overheating, or exposure to hot-spots altogether. Furthermore, there is a need for a fluid warming technique and apparatus that is more portable and does away with cartridges or components that break a closed sterilized system, minimizing risk of error or infection and avoiding safety and sterilization challenges presented by current means.
Therefore, there is a need in the art for a radio frequency fluid warmer and method that may be utilized to warm fluids, including IV fluids, which adequately addresses the problems with the prior art. It is to these ends that the present invention has been developed.